The present embodiments relate to a magnetic resonance tomography unit and to a method for operating the magnetic resonance tomography unit.
Magnetic resonance scanners are imaging devices that, for mapping an examination object, orient nuclear spins of the examination object using a strong external magnetic field and excite the nuclear spins to precession around this orientation via a magnetic alternating field. The precession or return of the spins from this excited state into a state with lower energy generates a magnetic alternating field (e.g., a magnetic resonance signal) as a response, which is received by antennae.
With the aid of magnetic gradient fields, a spatial encoding is impressed on the signals, and this subsequently enables allocation of the received signal to a volume element. The received signal is then evaluated, and a three-dimensional imaging representation of the examination object is supplied.
Magnetic alternating fields having a frequency corresponding to the Larmor frequency at the respective static magnetic field strength, and very high field strengths or outputs are provided for excitation of the precession of the spins. Antennae (e.g., local coils) that are arranged directly on the patient are used to improve the signal-to-noise ratio of the magnetic resonance signal received by the antennae.
From DE 102014213722, a magnetic resonance tomography unit that, for excitation of the spins in a plurality of regions of an examination object, has a transmitting unit with a plurality of segments, via which different high-frequency pulses having different frequencies are simultaneously emitted in each case to enable simultaneous image acquisition in a plurality of slices, is known.
The high-frequency pulses used for excitation have a power in the region of a few hundred watts to kilowatts to improve a signal-to-noise ratio (SNR). Legal limit values, which are denoted as the specific absorption rate (SAR), are defined to protect a patient from injury due to the effect of the heat of the high-frequency pulses. The speed of imaging using a magnetic resonance tomography unit is also limited by these limit values, which limit the energy absorption per unit of time and thereby the high-frequency power.